Recently, the IP (Internet Protocol) technique has been applied to various networks, and various kinds of problems, which were not conceivable at the creation of the Internet, have begun to occur.
Conventionally, in the Internet, in order to conform to a change in network topology, link information and the like are exchanged by using a routing protocol in each router in the network (see, for example, Japanese Patent Laid-Open Nos. 2003-046551 and 2003-273910).
When a given link breaks down, the routers at the two ends of the link notify each other of the breakdown of the link to allow each router in the network to update the route information, thereby inhibiting the use of the link.
If, however, there is a link which frequently undergoes a change in state, e.g., a wireless link, route information is frequency updated. Since the processing of updating route information is high in load, frequent execution of update processing will influence the performance of the router.
According to the Internet using packet-type communication models, a next transfer destination is determined from the destination in a packet header received by each router in the network, and a packet is transferred to the next transfer destination, thereby realizing end-to-end communication. It is therefore important for each router in the network to have correspondence information (route information) between a destination and the next transfer destination which corresponds to the state of the network at that point of time.
The respective routers in the network exchange reachability information such as the information of a link connected to the routers by using a routing protocol. Each router calculates route information from the reachability information obtained at this point of time, and performs transfer processing for packets by using the calculated route information.
FIG. 4 shows an example of the arrangement of the router. Referring to FIG. 4, a router 7 is connected to an external network (not shown) through the network interfaces 2-1 to 2-n. A routing protocol 5 exchanges reachability information with an adjacent router (not shown) through the network interfaces 2-1 to 2-n.
The routing protocol 5 stores acquired reachability information in an internal database 61 of a route calculation module 6. The route calculation module 6 calculates route information on the basis of the information in the internal database 61, and registers the route information in a route table 41 of a routing processing unit 4.
The routing processing unit 4 searches the route table 41 for a next transfer destination and a transmission interface by using the destination of the header of a packet received through network interfaces 2-1 to 2-n, and outputs a packet from the obtained transmission interface.
If, for example, the topology changes as the link connecting the routers is disconnected or the like, the router 7 notifies another router in the network of new reachability information by using the routing protocol 5.